Expected behavior

GPSampler should not slow down even after 100 trials.

Environment

• Optuna version: latest
• Python version: 3.12
• OS: Ubuntu 20.04

Error messages, stack traces, or logs

None

Steps to reproduce

Even the following single objective problem slows down significantly, but I do not know why.

import optuna


def objective(trial: optuna.Trial) -> tuple[float, float]:
    x = trial.suggest_float("x", -5, 5)
    return x**2


sampler = optuna.samplers.GPSampler(seed=0)
study = optuna.create_study(sampler=sampler)
study.optimize(objective, n_trials=110)

Additional context (optional)

No response