If it helps here is a git diff from slower to veryfast. Even if you just consider the ref value you can see how veryfast could be lower quality.
ref
In short, this value is the number of previous frames each P-frame can use
as references.
source
--- a/slower
+++ b/veryfast
@@ -1,20 +1,20 @@
cabac=1
-ref=8
+ref=1
deblock=1:0:0
-analyse=0x3:0x133
-me=umh
-subme=9
+analyse=0x3:0x113
+me=hex
+subme=2
psy=1
psy_rd=1.00:0.00
-mixed_ref=1
+mixed_ref=0
me_range=16
chroma_me=1
-trellis=2
+trellis=0
8x8dct=1
cqm=0
deadzone=21,11
fast_pskip=1
-chroma_qp_offset=-2
+chroma_qp_offset=0
threads=12
lookahead_threads=2
sliced_threads=0
@@ -25,17 +25,17 @@ bluray_compat=0
constrained_intra=0
bframes=3
b_pyramid=2
-b_adapt=2
+b_adapt=1
b_bias=0
-direct=3
+direct=1
weightb=1
open_gop=0
-weightp=2
+weightp=1
keyint=250
keyint_min=23
scenecut=40
intra_refresh=0
-rc_lookahead=60
+rc_lookahead=10
rc=crf
mbtree=1
crf=23.0
