Filter tcpdump file AFTER capturing
Yes, it is possible. You can use the following command:
tcpdump -r your_input_file.pcap -w your_output_file.pcap "your_filter"
Tcpdump will read the input file, apply the filter, and then write the output file. You need just to come up with the right filter.
Try netsniff-ng, it sequentially processes the pcap unlike Wireshark, which tries to load everything into RAM.