You didn't mention how that CPU usage is measured. I am assuming since it is a modern laptop with an i9, it is something like an i9-13900H, which has 14 cores and 20 threads (asymmetrical power architecture like ARM's big.LITTLE).
These processors are very fast at work that is well-parallelised. Computer scientists call this kind of work "embarrasingly parallel". Examples include video compression and decompression (as each macroblock can often be examined as one individual item), 3D raytracing on the CPU, compiling large packages of software and some types of file compression.
However, for applications that cannot be parallelised so well, they may exhibit little to no performance benefit on these processors. Circuit simulation does seem like something that would be hard to parallelise, because you might well have to wait for all of the results from one simulation iteration before the next is started. In fact in some cases higher parallelism can slow tasks down if the problem does not partition well.
Most of the performance improvements in modern computing have not come in the form of faster single threaded performance. There is no doubt that processors today are faster on single thread benchmarks, but the massive performance increases noticed are primarily in multithreaded workloads.
LTspice is not particularly good, in my experience, at using multiple threads. I just tried a test with the LT3496 example with threads set to maximum. It runs at around 1.1ms/sec, and CPU utilisation is 20%. I have a Ryzen 3800X, which is an older generation Zen architecture processor from AMD, it has 16 threads over 8 cores and runs at 4.1GHz normally. The CPU utilisation chart from Task Manager looks like this, it is clear that it is dominating four cores, but actually really only using two at any one time. To maximise performance the Windows kernel is swapping between cores to get the best benefit from the on-core cache (32KB I+D per core). But otherwise the other 12 threads I paid for are going unused. Shame! I set the max threads to 1, and LTspice is only barely slower: 1.02ms/sec vs 1.1ms/sec. This shows whatever benefits it gets from a multicore processor are minimal, at best.
To get this display you need to switch on the multiple CPU view in Task Manager. By default it only shows one CPU.
This is not a memory limit, this is entirely a CPU limit, but you have hit the single threaded CPU limit of the laptop I expect.
Edit: corrected error, re-read your post.