I believe I may have located a possible source of this problem.
Due to recent changes, the plugin queries the APIs for CoinGecko and CoinMarketCap in a different, better way. This however is possibly causing a large amount of data to be downloaded in certain cases. This data is saved in the DB even in most cases when it does not need to be saved.
If possible, can you try editing the file includes/rates.php, and change line 712 from:
if ( is_string( $result ) ) {
to
if ( is_string( $result ) && $cache_seconds ) {
If I am correct then this should make a huge difference in memory usage. If someone can give it a try I would like to know if it solved the problem.
If not, we will have to see in the next release, which will show debug info on memory statistics.