I don't recall anything about Env-Handler.
It's been awhile since I tweaked the startup-sequence or user-startup, but:
By supressing output you can leave the video output off till it's done booting. By this I mean you don't want any text output because this would require the chipset to start DMA and that can make the 680X0 compete for access to custom-chip ram. IIRC there are somethings I like output with so I leave them for last items in user-startup.
Just add a > Nil: to the end of any line whose output you want supressed.
If my memory is wrong I'm sure somebody will refresh it. Also I don't think my AGA WB screen was ever more than 8 colors. Did that to save chipram, but it can also help make things faster than a 128 color WB screen. Especially if you're running at 1440x486.
I also still use many of the ARP commands because they're smaller, load faster, and since they're 100% assem, they're much faster.
PFS and SFS are faster filesystems but I think they really use a lot of DMA in a more hoglike fashion? Somebody else can comment on that. This can be good or depend depending. Overall, PFS and SFS are good things. Real good.
You can also make use of the recoverable ramdisks out there and adjust your startup files to check to see if they're in use, and if so, boot from there. This makes the 1st boot a tad slower, but later reboots much faster.