I reckon this user David Anderson had the answer, but nothing has improved in 10 years as far as I can see:

https://stackoverflow.com/questions/2103968/gvim-runs-very-slowly-when-editing-files-on-a-windows-share

"I use Process Monitor from SysInternals.com and it revealed bad behavior when attempting to open "\server\TestTool\foo\ReadMe.TXT"

It first attempts a CreateFile (aka, Directory open) on "\serve". Notice the last character is missing. This will cause 4 seconds to time out with "OBJECT PATH INVALID".

Then it tries CreateFile on "\server\TestToo". Server name is correct by the last letter of "TestTool" is clipped. Again, a 3 second time out with "BAD NETWORK NAME".

Finally it gets it right and calls CreateFile on "\server\TestTool" which works right away. Then CreateFile on "\server\TestTool\foo" which works right away. Then CreateFile on "\server\TestTool\foo\ReadMe.TXT" which works right away."

"I downloaded the source code and found the problem. They are using FindFirstFile() which is not supposed to be used on root directories or network shares. Bug in vim."

I reckon David Anderson had the answer, but nothing has improved in 10 years as far as I can see:

https://stackoverflow.com/questions/2103968/gvim-runs-very-slowly-when-editing-files-on-a-windows-share

David's answer (with a small edit):

… I use Process Monitor from SysInternals.com and it revealed bad behavior when attempting to open "\server\TestTool\foo\ReadMe.TXT"

It first attempts a CreateFile (aka, Directory open) on "\serve". Notice the last character is missing. This will cause 4 seconds to time out with "OBJECT PATH INVALID".

Then it tries CreateFile on "\server\TestToo". Server name is correct by but the last letter of "TestTool" is clipped. Again, a 3 second time out with "BAD NETWORK NAME".

Finally it gets it right and calls CreateFile on "\server\TestTool" which works right away. Then CreateFile on "\server\TestTool\foo" which works right away. Then CreateFile on "\server\TestTool\foo\ReadMe.TXT" which works right away.