Vaadin Flow has built-in loading indicator which is shown when synchronous requests take long to handle. Normally it is shown on the top. If you want to show it prominently in the middle of the screen, use this CSS.

connection-indicator.css

vaadin-connection-indicator {
    position: fixed;
    z-index: 251;
    top: 50%;
    width: 20%;
    left: 40%;
}

.v-loading-indicator {
    position: unset !important;
    height: 10px;
    z-index: unset !important;
    left: unset !important;
    top: unset !important;
    right: unset !important;
    background: var(--lumo-primary-color-50pct);
    border-radius: var(--lumo-border-radius-m);
    border: 1px solid var(--lumo-primary-color);
}