Depending on the overall architecture of your network and how you have configured the IDS/IPS system you will not be able to determine the presence based on performance degradation. Most systems are set up so that traffic is mirrored to the IDS/IPS port and do not sit in-line of data flows between host client and service offered. Degradation of service due to a properly implemented IDS is highly unlikely.
BUT there is one way to detect whether an IDS/IPS system is in place if it contains a specific feature... Run a sniffer trace alongside of your scan and focus on a specific aspect in TCP - window size. Once detected certain IDS/IPS systems will negotiate a small window size to tar pit your session to render your scan useless. I have my IDS system to negotiate a 1 byte window size
So... if you're scanning a network and evasion is your goal, you tailor the scan to come in below the suspected thresholds. You have time on your side, so why the rush