macOS Startup Manager is very slow and laggy
I am suffering from same issue since December.

This is a hardware error, unfortunately it is hard to prove it to apple care. They will try os reload, factory reset ... before they realize it is hardware error.

Same issue here, noticed it in FileVault, but then realized it is there even on Boot selection section or even Diagnosis section. Disabling FileVault won't solve the issue, it will just cover it up.

I wasted a lot of time with software sides of things, reinstalling OS and all. they all didn't help. It is a Hardware issue.

I realized I have this issue once I started using SD Card reader for storing my files and extending my 128gb MBP. If SD Cards goes in, even for 1 second, and then take it out, boot is going to be slow until I do a NVRAM reset. Apple care didn't find any issue first time but second time they admit there is a hardware issue in my Macbook and they replaced it. I will report back issue returned.

You need to bring it to Apple care, and describe the issue in detail. I had to print a A4 paper with a detailed description (told them issue happen when put in a sd card and then take it out) and screenshot of diagnosis page showing SD Card error in diagnosis boot page (because for some reasons, it was randomly showing that error and apparently when I give my Macbook for first repair, it passed test 100% successfully) Reception who receive my Macbook, she didn't really reflect things I told to her to technician. She even typed issue of my macbook as "Booth up slow", you get the idea.

There are many users like us. I guess there are many other users who have this issue but are not aware of it. they just think it is normal to wait 30 to 60 seconds for boot and they never use filevault or boot option menu. I have found some here, this are all from SAME underlying issue:


macOS Sierra FileVault enabled freezing at login
macOS Sierra boot problem related to Apple SD Card reader driver
macOS Sierra slow to accept password at login
Whenever I enable FileVault, OSX is super slow to boot. I never had this problem before and don't know what changed. 
OSX Startup Manager (Boot drive selection screen) slow/un-responsive in Macbook Pro (early 2015)
OSX Boot drive selection screen is slow, un-responsive and takes forever to load in Macbook Pro (early 2015)