Will iOS 10 take up more space on my IPhone 6S (64gb) than the current iOS 9.3.4 that I'm on?

Should I update or will it take up a load of space and slow down my phone?

Updates usually take up more space

Slightly more but that won't affect anything.

IOS updates these days usually are taking up less space initially. Even so, it wouldn't drastically reduce the storage available & neither shouldn't slow it down.