diff --git a/src/float/fast_float/ascii_number.cr b/src/float/fast_float/ascii_number.cr
index a6bb00cdd91c..1c4b43ea4b7d 100644
--- a/src/float/fast_float/ascii_number.cr
+++ b/src/float/fast_float/ascii_number.cr
@@ -135,8 +135,8 @@ module Float::FastFloat
       p += 1
     end
     end_of_integer_part = p
-    digit_count = (end_of_integer_part - start_digits).to_i64!
-    answer.integer = Slice.new(start_digits, digit_count.to_i32)
+    digit_count = (end_of_integer_part - start_digits).to_i32!
+    answer.integer = Slice.new(start_digits, digit_count)
     if fmt.json_fmt?
       # at least 1 digit in integer part, without leading zeros
       if digit_count == 0
@@ -162,7 +162,7 @@ module Float::FastFloat
         i = i &* 10 &+ digit # in rare cases, this will overflow, but that's ok
       end
       exponent = before - p
-      answer.fraction = Slice.new(before, (p - before).to_i32)
+      answer.fraction = Slice.new(before, (p - before).to_i32!)
       digit_count &-= exponent
     end
     if fmt.json_fmt?
@@ -246,7 +246,7 @@ module Float::FastFloat
         int_end = p + answer.integer.size
         minimal_nineteen_digit_integer = 1000000000000000000_u64
         while i < minimal_nineteen_digit_integer && p != int_end
-          i = i &* 10 &+ (p.value - '0'.ord).to_u64!
+          i = i &* 10 &+ (p.value &- '0'.ord).to_u64!
           p += 1
         end
         if i >= minimal_nineteen_digit_integer # We have a big integers
@@ -255,7 +255,7 @@ module Float::FastFloat
           p = answer.fraction.to_unsafe
           frac_end = p + answer.fraction.size
           while i < minimal_nineteen_digit_integer && p != frac_end
-            i = i &* 10 &+ (p.value - '0'.ord).to_u64!
+            i = i &* 10 &+ (p.value &- '0'.ord).to_u64!
             p += 1
           end
           exponent = (answer.fraction.to_unsafe - p) &+ exp_number
diff --git a/src/float/fast_float/bigint.cr b/src/float/fast_float/bigint.cr
index 5783b3454b7f..14b0bb2d0549 100644
--- a/src/float/fast_float/bigint.cr
+++ b/src/float/fast_float/bigint.cr
@@ -250,9 +250,12 @@ module Float::FastFloat
   # multiply bigint by scalar value.
   def self.small_mul(vec : Stackvec(Size)*, y : Limb) : Bool forall Size
     carry = Limb.zero
-    vec.value.map_with_index! do |xi, i|
+    i = 0
+    while i < vec.value.size
+      xi = vec.value.unsafe_fetch(i)
       z, carry = scalar_mul(xi, y, carry)
-      z
+      vec.value.unsafe_put(i, z)
+      i &+= 1
     end
     if carry != 0
       fastfloat_try vec.value.try_push(carry)
@@ -270,8 +273,10 @@ module Float::FastFloat
     end
 
     carry = false
-    y.each_with_index do |yi, index|
+    index = 0
+    while index < y.size
       xi = x.value.unsafe_fetch(index &+ start)
+      yi = y.unsafe_fetch(index)
       c2 = false
       xi, c1 = scalar_add(xi, yi)
       if carry
@@ -279,6 +284,7 @@ module Float::FastFloat
       end
       x.value.unsafe_put(index &+ start, xi)
       carry = c1 || c2
+      index &+= 1
     end
 
     # handle overflow
@@ -440,7 +446,8 @@ module Float::FastFloat
       elsif @vec.size < other.value.@vec.size
         -1
       else
-        @vec.size.downto(1) do |index|
+        index = @vec.size
+        while index > 0
           xi = @vec.unsafe_fetch(index &- 1)
           yi = other.value.@vec.unsafe_fetch(index &- 1)
           if xi > yi
@@ -448,6 +455,7 @@ module Float::FastFloat
           elsif xi < yi
             return -1
           end
+          index &-= 1
         end
         0
       end
@@ -464,10 +472,12 @@ module Float::FastFloat
       shl = n
       shr = LIMB_BITS &- n
       prev = Limb.zero
-      @vec.map_with_index! do |xi, index|
-        (xi.unsafe_shl(shl) | prev.unsafe_shr(shr)).tap do
-          prev = xi
-        end
+      index = 0
+      while index < @vec.size
+        xi = @vec.unsafe_fetch(index)
+        @vec.unsafe_put(index, xi.unsafe_shl(shl) | prev.unsafe_shr(shr))
+        prev = xi
+        index &+= 1
       end
 
       carry = prev.unsafe_shr(shr)