diff --git a/soroban-sdk/src/num.rs b/soroban-sdk/src/num.rs
index e96b729af..7a5b59b75 100644
--- a/soroban-sdk/src/num.rs
+++ b/soroban-sdk/src/num.rs
@@ -2,8 +2,8 @@ use core::{cmp::Ordering, convert::Infallible, fmt::Debug};
 
 use super::{
     env::internal::{
-        DurationSmall, DurationVal, Env as _, I256Small, I256Val, TimepointSmall, TimepointVal,
-        U256Small, U256Val,
+        DurationSmall, DurationVal, Env as _, I256Object, I256Small, I256Val, TimepointSmall,
+        TimepointVal, U256Object, U256Small, U256Val,
     },
     Bytes, ConversionError, Env, TryFromVal, TryIntoVal, Val,
 };
@@ -227,10 +227,9 @@ impl U256 {
     }
 
     pub fn from_u128(env: &Env, u: u128) -> Self {
-        let lo: Bytes = Bytes::from_array(env, &u.to_be_bytes());
-        let mut bytes: Bytes = Bytes::from_array(env, &[0u8; 16]);
-        bytes.append(&lo);
-        Self::from_be_bytes(env, &bytes)
+        let lo_hi = (u >> 64) as u64;
+        let lo_lo = u as u64;
+        Self::from_parts(env, 0, 0, lo_hi, lo_lo)
     }
 
     pub fn from_parts(env: &Env, hi_hi: u64, hi_lo: u64, lo_hi: u64, lo_lo: u64) -> Self {
@@ -254,14 +253,23 @@ impl U256 {
     }
 
     pub fn to_u128(&self) -> Option<u128> {
-        let be_bytes = self.to_be_bytes();
-        let be_bytes_hi: [u8; 16] = be_bytes.slice(0..16).try_into().unwrap();
-        let be_bytes_lo: [u8; 16] = be_bytes.slice(16..32).try_into().unwrap();
-        if u128::from_be_bytes(be_bytes_hi) == 0 {
-            Some(u128::from_be_bytes(be_bytes_lo))
-        } else {
-            None
+        let v = *self.val.as_val();
+
+        // If v is U256Small it can be converted directly
+        if let Ok(small) = U256Small::try_from(v) {
+            return Some(u64::from(small) as u128);
+        }
+
+        // Otherwise use U256Object and take low sections if high are empty
+        let obj: U256Object = v.try_into().ok()?;
+        let hi_hi = self.env.obj_to_u256_hi_hi(obj).unwrap_infallible();
+        let hi_lo = self.env.obj_to_u256_hi_lo(obj).unwrap_infallible();
+        if hi_hi != 0 || hi_lo != 0 {
+            return None;
         }
+        let lo_hi = self.env.obj_to_u256_lo_hi(obj).unwrap_infallible();
+        let lo_lo = self.env.obj_to_u256_lo_lo(obj).unwrap_infallible();
+        Some(((lo_hi as u128) << 64) | (lo_lo as u128))
     }
 
     pub fn to_be_bytes(&self) -> Bytes {
@@ -474,16 +482,14 @@ impl I256 {
     }
 
     pub fn from_i128(env: &Env, i: i128) -> Self {
-        let lo: Bytes = Bytes::from_array(env, &i.to_be_bytes());
-        if i < 0 {
-            let mut i256_bytes: Bytes = Bytes::from_array(env, &[255_u8; 16]);
-            i256_bytes.append(&lo);
-            Self::from_be_bytes(env, &i256_bytes)
+        let lo_hi = (i >> 64) as u64;
+        let lo_lo = i as u64;
+        let (hi_hi, hi_lo) = if i < 0 {
+            (-1_i64, u64::MAX) // sign extend 1 bit
         } else {
-            let mut i256_bytes: Bytes = Bytes::from_array(env, &[0_u8; 16]);
-            i256_bytes.append(&lo);
-            Self::from_be_bytes(env, &i256_bytes)
-        }
+            (0_i64, 0_u64) // sign extend 0 bit
+        };
+        I256::from_parts(env, hi_hi, hi_lo, lo_hi, lo_lo)
     }
 
     pub fn from_parts(env: &Env, hi_hi: i64, hi_lo: u64, lo_hi: u64, lo_lo: u64) -> Self {
@@ -507,13 +513,27 @@ impl I256 {
     }
 
     pub fn to_i128(&self) -> Option<i128> {
-        let be_bytes = self.to_be_bytes();
-        let be_bytes_hi: [u8; 16] = be_bytes.slice(0..16).try_into().unwrap();
-        let be_bytes_lo: [u8; 16] = be_bytes.slice(16..32).try_into().unwrap();
-        let i128_hi = i128::from_be_bytes(be_bytes_hi);
-        let i128_lo = i128::from_be_bytes(be_bytes_lo);
-        if (i128_hi == 0 && i128_lo >= 0) || (i128_hi == -1 && i128_lo < 0) {
-            Some(i128_lo)
+        let v = *self.val.as_val();
+
+        // If v is I256Small it can be converted directly
+        if let Ok(small) = I256Small::try_from(v) {
+            return Some(i64::from(small) as i128);
+        }
+
+        // Otherwise use I256Object and take low sections if high are either
+        // all 1 bits or all 0 bits (negative and positive respectively)
+        let obj: I256Object = v.try_into().ok()?;
+        let hi_hi = self.env.obj_to_i256_hi_hi(obj).unwrap_infallible();
+        let hi_lo = self.env.obj_to_i256_hi_lo(obj).unwrap_infallible();
+        let lo_hi = self.env.obj_to_i256_lo_hi(obj).unwrap_infallible();
+        let lo_lo = self.env.obj_to_i256_lo_lo(obj).unwrap_infallible();
+        // The low 128 bits as an i128
+        let lo = (((lo_hi as u128) << 64) | (lo_lo as u128)) as i128;
+
+        if lo < 0 && hi_hi == -1 && hi_lo == u64::MAX {
+            Some(lo) // if negative low, then high must be all 1 bit
+        } else if 0 <= lo && hi_hi == 0 && hi_lo == 0 {
+            Some(lo) // if non-negative low, then high must be all 0 bit
         } else {
             None
         }