If you are using Catalog Search APIs for any customer-facing features, you are doing it wrong!
I have seen this problem a couple of times – the search feature on the site is “dead” – it is very slow, and the log file is usually filled with dead lock or timeout error. As it turns out, the search feature was implemented by Catalog Search APIs, which is a big no-no.
To be clear, there are two builtin APIs related to searching in Episerver Commerce: the “fast” one, which can be done via SearchManager, ISearchCriteria and ISearchResults, is the SearchProvider APIs. It’s the indexed search (strictly speaking, you can make it not “indexed”, but that’s beside the point), and the actual search functions will be provided by providers, like LuceneSearchProvider, Solr35SearchProvider, or FindSearchProvider.