P: Long AVCHD video files are causing hanging issues in premiere pro

So I'm going to preface this by saying that I do multi track editing in a weird way; I layer all the video files, and then cut down through the layers to show different camera angles. On a project I'm currently working on I have an .mts file as the bottom layer (avchd), and 3 mp4 files above that. Recently Premiere pro has started exhibiting strange behaviour when playing back the long mts / avchd file. If I make a cut or make any change to the mts video file, premiere hangs and appears to be reindexing the video file. During this time the play button doesn't work - and worse, if playback is mid progress, I'm then unable to stop playback until it's finished reindexing the file. This slow down results in very lengthy edit times.

This wasn't a problem until one of the more recent updates. Using disk management, you can see premiere pro maxing out the USB hard drive's transfer speed trying to re-read the entire video file which I'm trying to work on. Toggling visibility on the layer containing the mts file fixes the issue, but while this does speed up the progress until I need that layer, it doesn't get around the fact that premiere pro still can't seem to handle avchd/mts files at the moment.

Unfortunately I'm unable to share footage as it's of a sensitive nature with child protection and blurring needing to be done on the finished product.

The file in question is from a sony handycam, 1080p 50fps, 24mb/s 256kbs audio, 1hr 3 mins in length and about 11.8gb

Any insight on this would be appreciated as almost all of my multicam projects use MTS files for at least one of the cameras.