Convert saved evtx files to text
In the end I went with Log Parser to convert to CSV and then [System.IO.File]::ReadLines($filename) to search through the text. An 800MB .evtx file can be converted in about 2 min 30 sec and then reading through the file takes about 2 mins. Possibly it could be quicker exporting to XML or into a database but it will do for me with the amount of time I had to spend.
$logparser = "c:\program files (x86)\Log Parser 2.2\logparser.exe"
$query = "SELECT * INTO c:\logs\logs.csv FROM c:\logs\logs.evtx"
& $logparser -i:evt -o:csv $query