Whilst I've been building part of my map in the air, about 63 blocks from 1 bedrock layer (on superflat), Minecraft usually freezes for a second or two when I have one block left to place, like on the picture I've attached.

I allocated a bit more RAM (1GB min, 1.5GB max) to see if that would help, but it still freezes for a second or two. So then I used the debug command, below is what i got on the "Game Output" from the launcher:

[17:50:28] [Server thread/INFO]: [DTaylorMedia: Started debug profiling]
[17:50:28] [Client thread/INFO]: [CHAT] Started debug profiling
[17:50:36] [Server thread/WARN]: Something's taking too long! 'root.jobs' took aprox 1026.975293 ms
[17:50:36] [Server thread/WARN]: Something's taking too long! 'root' took aprox 1029.666945 ms
[17:50:43] [Server thread/INFO]: [DTaylorMedia: Stopped debug profiling after 15.45 seconds (309 ticks)]
[17:50:43] [Client thread/INFO]: [CHAT] Stopped debug profiling after 15.45 seconds (309 ticks)